1. Field of the Invention
The present invention relates to a driving circuit for a plurality of cascade light emitting diodes, and more particularly, relates to a driving circuit having different total number of LED units which can be illuminated in accordance with various voltage changes, and the driving circuit for the cascade light emitting diodes having increased efficiency.
2. Description of the Prior Art
Referring to FIG. 1, a conventional light emitting diode (LED) driver circuit is illustrated. For reducing circuit sizes and in light of cost saving concerns, some manufacturers have adopted the usage of transistor control circuit in specified voltage interval under conducting state, so as to omit having larger elements, such as, transformer and filter capacitor.
The LED driver circuit 10 has an alternating current power supply 12 with a bridge rectifier comprising a plurality of diodes 141, upon which rectifying is performed to form a direct current power source. In the LED driver circuit, an N-type MOS power transistor 16 is used to control on and off of electric current flow, a first transistor 151 is used to limit the current flow for the light emitting diode 18, and a second transistor 153 is used to control the duration of the electrical current flow.
The drain of the power transistor 16 is coupled to the bridge rectifier 14, the source is coupled to the base of the first transistor 151, and the gate is coupled to the collector of the first transistor 151. The light emitting diode 18 is serially connected between the emitter of the first transistor 151 and the bridge rectifier 14. In addition, a first resistor 171 is connected between the drain and gate of the power transistor 16. The fifth resistor 175 is connected between the base and the emitter of the first transistor 151. The sixth resistor 176 is connected between the collector and the base of the first transistor 151.
The second resistor 172 and the third resistor 173 are serially connected between the drain of the power transistor 16 and the emitter of the first transistor 151. The base of the second transistor 153 is connected to the connection point of the second resistor 172 and the third resistor 173, and the emitter of the second transistor 153 is connected to the emitter of the first transistor 151. The fourth resistor 174 is connected to between the collector of the second transistor 153 and the gate of the power transistor 16. Furthermore, a capacitor 155 is coupled to the two ends of the light emitting diodes 18.
According to the above mentioned device configuration, when the voltage output from the bridge rectifier 14 is slowly increased from zero, the gate voltage of the power transistor 16 is correspondingly increased accordingly. When the voltage difference between the gate and source becomes larger than the threshold voltage, the power transistor 16 starts to conduct, and the current flowing through the fifth resistor 175 starts to increase. When the potential difference of the electrical current flowing through the fifth resistor 175 is larger than the threshold voltage of the first resistor 151, the first resistor 151 starts conducting, and at this moment, the gate voltage of the power transistor 16 is pulled down to a reduced voltage level, thereby reducing the conducting current. The reduction of the conducting current of the power transistor 16 then leads to the reduction of the potential/voltage difference through the fifth resistor 175, thereby causing the degree of conduction of the first transistor 151 to be reduced, and the reduction of the degree of pull down for the gate voltage of the power transistor 16 also occurs. As a result, the conducting current for the power transistor 16 would again increase, thereby mutually restraining and limiting the current flow through the fifth resistor 175, and making it thereof becoming a fixed value.
As the output voltage for the bridge rectifier 14 increases, the current flowing through the second resistor 172 and the third resistor 173 is slowly increased, finally making the second transistor 153 conducting and pulling down the gate voltage of the power transistor 16, thereby turning off the power transistor 16. When the voltage output of the bridge rectifier 14 is slowly reduced from a high voltage level, the current of the third resistor 173 is slowly reduced; when the potential/voltage difference for the current flow through the third resistor 173 is lower than the threshold voltage of the second transistor 153, the second transistor 153 is then turned off. When the gate voltage of the power transistor 16 is increased, the power transistor 16 is thereby allowed to be conducting, and again the driving current is provided to the LED 18. Finally, the output voltage of the bridge rectifier 14 is reduced to zero, and the entire circuit is returned to zero current flow state, and thereby completing one cycle.
Although the aforementioned conventional circuit can achieve objects such as the omission of transformer and/or filter capacitor; however, as seen in the voltage waveform, for the sake of preventing the LED 18 from burning out, the above conventional circuit can only be conducting within a small limited voltage range, whereas in other voltage ranges, it is configured in an off state, thereby leading to excessively low energy utilization rate.
Meanwhile, for improving energy utilization rate, US patent application publication number 20090230883 disclosed a stacked LED controller, in which each LED controller drives one or more LEDs, respectively, and can serially connect a string of LED controllers between a supply voltage source and ground.
When an LED controller detects that its input voltage is below a threshold voltage needed for driving the LED and thus cannot drive an upstream LED controller, a bypass switch is used to bypass an adjacent upstream controller depending on the detected input voltage level. When the input voltage exceeds a threshold needed for driving the LED, all of the normally-on bypass switches are turned off, so that all of the upstream controllers are energized.
The aforementioned LED controller although may improve power usage efficiency, but their corresponding circuitry is relatively complicated, and their manufacturing cost is relatively high, and the voltage without reaching the threshold voltage of the most upstream LED controller can still lead to having some electric power to be wasted.